Generally, a ring rolling process is a process which machines a seamless ring in a continuous manner into a predetermined size, thus producing a product, that is, a rolled ring product. Such ring rolling processes are used to manufacture ring parts used in a variety of fields, for example, power generation equipment, chemical plants, gas turbines, jet engines, etc.
Compared to a ring forging process which is different from a rolling process, advantages of the ring rolling process include that the working speed is rapid, the temperature can be maintained, the production yield can be enhanced, and so on. Particularly, in the case of a rolled ring product that is manufactured by a ring rolling process, the grain flow line is continuously formed in the circumferential direction of the product, thus providing superior characteristics.
FIG. 1 is of views showing an entire ring rolling process. A method of manufacturing a rolled ring product with the ring rolling process will be explained with reference to FIG. 1. At step S1, an initial billet 1 with, for example, a cylindrical structure, is prepared by cutting off a raw billet to an appropriate size using gas cutting or a machine saw.
Subsequently, at step S2, a heating furnace 2 heats the initial billet 1 to the desired temperature. At step S3, the heated initial billet 1 is transferred to a forging press 3.
A mold of the forging press 3 that has been preheated upset-forges the heated initial billet 1, thus pressing the initial billet 1 in the axial direction, at step S4.
Thereafter, at step S5, a punch 4 pierces an intermediate product 1a that has been compressed by upset-forging the initial billet 1, thus forming a hollow blank 9.
At step S6, a ring rolling machine subsequently ring-rolls the blank 9. The ring rolling machine includes a main roll 5 which presses a circumferential outer surface of the blank 9, a pressure roll 6 which presses a circumferential inner surface of the blank 9, an upper axial roll 7 which presses an upper surface of the blank 9, a lower axial roll 8 which presses a lower surface of the blank 9, and a plurality of guide rolls 10 which rotatably support the circumferential outer surface of the blank 9. This ring rolling process produces a rolled ring product 11 into a predetermined shape, at step S7.
Particularly, a flange, which is used to connect tubes that form the framework of a wind tower, is typically manufactured by such a ring rolling process. FIG. 2 is a sectional view of a typical flange for wind towers.
As shown in FIG. 2, the flange 10 for wind towers includes a connection part 10a that protrudes from the body of the flange 10 and is used when welding a corresponding tube to the flange 10. Producing the flange 10 includes the ring rolling process manufacturing a ring having a rectangular cross-section as illustrated in FIG. 1, and post-processing the ring, thus producing a final product.
Recently, a method is used in which an intermediate product having a depression in a circumferential inner surface thereof is formed, the intermediate product is cut into two parts at a medial portion thereof corresponding to the depression, and then each of the two parts is post-processed, thus forming a final product 10, for example, such as those shown in FIG. 3.
Representative examples of the above conventional technique were proposed in Korean Patent Application No. 10-2009-0131482 (filed on Dec. 28, 2009: Semi-finished ring rolling machine and method of manufacturing semi-finished ring using the same), Patent Application No. 10-2010-0007954 (filed on Jan. 28, 2010: Apparatus and method for manufacturing asymmetric large ring), etc.
However, to form such a product having a depression, a protrusion must be provided on the pressure roll.
In the case of a flange for power towers having an asymmetric cross-section, the material utilization ratio is reduced, and it takes a comparatively long amount of time to carry out a post process. Furthermore, if it is necessary to replace a pressure roll with a pressure roll provided with a protrusion to form an asymmetric cross-section in a blank, it further increases the time it takes to perform the entire machining process.